1. Field of the Invention
The present invention relates to a semiconductor device and the method of manufacture and, particularly, relates to a semiconductor device having memory functions.
2. Description of Related Art
As one type of nonvolatile memory, a floating gate nonvolatile memory is conventionally known. This memory demonstrates nonvolatile memory functions by injecting carriers to a floating gate and holding therein.
With this type of nonvolatile memory, the floating gate EPROM at a p-channel having an MOS structure was first used for practical purposes. For this type of floating gate, polycrystalline silicon doped with a large quantity of impurities is used, and carriers are injected to floating gates (for writing or programming) by causing avalanche breakdown at drain junctions. This type of nonvolatile memory is called FAMOS (Floating-gate Avalanche-injection MOS). The information written in FAMOS can be erased by the irradiation of ultraviolet rays and X-rays at a sufficiently high energy level.
Nonvolatile memory, having a structure wherein a control gate made of polycrystalline silicon is laminated on the FAMOS floating gate, is called SAMOS (Stacked-gate Avalanche-injection MOS) memory. An appropriate level of voltage is applied to the control gate during the injection process of carriers for avalanche breakdown, so that an electric field near a drain is intensified and avalanche breakdown is likely to occur. At the same time, the electrons generated by the avalanche breakdown may be more effectively attracted to the side of a floating gate, thereby shortening the writing time. Additionally, the control gate may be used like the gate electrodes of normal MOS transistors during the process of information readout.
Devices having an SAMOS structure at an n-channel have been recently referred to as FAMOS, and have become the standard EPROM structure. In this case, channel hot electrons are injected into a floating gate.
Furthermore, according to other research, MOS memory has been proposed as in the thesis, xe2x80x9cMOS Memory Using Germanium Nanocrystals Formed by Thermal Oxidation of Sil-xGex, Ya-Chin King et al., IEDM 98 115-118.xe2x80x9d This is a memory element in which a charge trapping bodies comprising germanium fine particles are buried in an MOSFET gate insulating body. On the other hand, since economical glass substrates, instead of expensive quartz substrates, may be used and preferable TFT characteristics may be easily obtained, the polysilicon TFT formed in the process of a relatively low temperature (about 600xc2x0 C. or below) has been focused upon.
However, although this TFT is used for the picture elements of displays and peripheral circuits, it is not a device that could be used as a memory element like the above-noted MOS memory. Therefore, a memory and a display cannot be mounted on one panel in one body in, for example, an active matrix display in which a TFT is used for a picture element unit. This is one of the obstacles to the further miniaturization and electricity reduction of liquid crystal display devices or the like.
It is an object of the present invention to provide memory functions to a semiconductor device and to broaden the application of semiconductor devices.
In order to achieve the above objective, a semiconductor device according to the present invention has a substrate, a semiconductor layer, in which each source, channel and drain region is formed on the substrate, and an insulating film is formed on the semiconductor layer, and granular electron trapping bodies are placed inside the insulating film to trap the electrons of injected carriers.
Preferably, the electron trapping bodies are a plurality of semiconductor or metal granules. For instance, the plurality of granules is made up of silicon particles. It is preferable that these silicon particles have a diameter of 1 xcexcm or less, 1000 angstroms or less, or 500 angstroms or less.
Moreover, it is preferable that the insulating film comprise a first insulating film formed on the semiconductor layer and a second insulating film formed on the first insulating film, with the plurality of granules being placed between the first insulating film and the second insulating film. In this case, the first insulating film is preferably formed at an extremely thin thickness. Preferably, the first insulating film is formed so as to have a film thickness of 500 angstroms or less, 100 angstroms or less, or 50 angstroms or less.
More preferably, in the above-mentioned structure, a control gate for electrical field application is formed on the insulating film so as to face the channel region.
It is further preferable that the transistor, formed of the substrate and the semiconductor layer mentioned above, be a thin-film transistor (TFT). For example, the semiconductor layer is formed in a low-temperature polysilicon process, and the thin-film transistor is formed as low-temperature polysilicon TFT.
On the other hand, the manufacture of a semiconductor device according to the present invention includes a first step of forming a semiconductor layer, which has each source, channel and drain region on a substrate; and a second step of forming an insulating body, which has granular charge trapping bodies inside, to trap the charge of injected carriers.
In this case, it is preferable that the second step also have the steps of forming a first insulating film, constituting a portion of the above-mentioned insulating film, on the semiconductor layer; placing the granular charge trapping bodies on the first insulating film; and forming a second insulating film, constituting the remaining portion of the insulating film mentioned above, on the first insulating film with the charge trapping bodies being kept on the first insulating film.
Preferably, the first insulating film is formed by plasma oxidation. Additionally, as another preferred example, charge trapping bodies are formed by sputtering and etching. In this case, it is preferable that Alxe2x80x94Si be sputtered and that only Al be etched thereafter. Moreover, according to another preferred example, the second insulating film is formed by the CVD method. Besides these methods, it is also possible to form the first insulating film by plasma oxidation, the charge trapping bodies by sputtering and etching, and the second insulating film by the CVD method.
Furthermore, the granular charge trapping bodies are, for instance, silicon particles.
Moreover, the first step is a step where the semiconductor layer is formed in a low-temperature polysilicon process. A low-temperature polysilicon TFT (thin-film transistor) may be formed in this step.
As explained above, the present invention has a semiconductor layer, which has each source, channel and drain region formed on a substrate, an insulating film formed on the semiconductor layer, and granular charge trapping bodies (for instance, a plurality of semiconductor or metal particles) inside the insulating film to trap the charge of injected carriers, so that TFT memory may be provided by adding memory functions with granular charge trapping bodies to a thin-film transistor (TFT) element comprising a substrate and a conductive layer.
As a result, the applications of TFT elements are broadened to memory elements. Conventionally, TFT elements are only applied to the picture elements of displays and the peripheral circuits. As the TFT memory is used as a memory unit, the unit may be mounted on the same panel as that of other thin-film structural bodies using TFTs (for instance, a liquid crystal display and the driver circuit thereof), thus significantly miniaturizing, compacting or making an energy-saving device and system.